Sensor picks up specific physical or chemical properties to complete action. To be measured parameter must be predefined either qualitatively or Quantitatively. Sensor not always points to an electrical or electronic component, but also biological component. This article on Sensor is directly related to Robotics, Cyborg etc. In fact the Senors used in technology are very primitive when compared with biological senors.
Sensor in Technology
A Sensor is a technical component, which picks specific physical or chemical properties (like heat, temperature, humidity, pressure, sound, brightness or acceleration ), through physical or chemical effects and further processed to transfer it to other part of the system it is attached to be human readable or readable by a computer.
Example of a simplest sensor is a Thermometer. The Mercury within the bulb and the tube is the Sensor for measurement of Temperature. Heat can not be measured directly and needs more complex technical parts or can be measured using basic Laws of Heat of Physics.
Active Sensor :
Active sensor generate an electrical signal such as electrodynamic or piezoelectric flow. It needs no electrical supply, so in view of usage of energy they are passive sensors. A classic exception is the thermocouple which constantly generates a differential voltage.
Passive Sensor :
Passive sensor includes passive components , whose parameters are changed by the measure. By a primary electronics, these parameters are converted into electrical signals. Here, an externally supplied power supply is needed. Classic example is resistance thermometer.
Modern sensors often have extensive secondary electronics parts which are operated by externally supplied energy. However, not all of these sensors are active sensors.
For easiness of understanding of Sensor, we have deliberately used Physical Character like Temperature in this article on Sensor. As we can mathematically measure all the variable (s) and constant (s) associated with a Sensor, with can be expressed mathematically either as equation of Calculus (Dy/Dx) or can plot as graph.
Sensor in Biological Science
At molecular level, Sensor in biological system is equivalent to the receptors. A biosensor is an analytical or application device which uses the logics and mechanisms of the particular Sensor (=receptor) in biological system in vitro. When used in vitro they are usually referred as Sensor too.
It is not possible to create a hypothesis on an unknown sensor when the gross working principle is not known or ill defined. Example can be, presence of unknown mechanism among animals to detect natural disasters that will happen in future. Smell has relatively known mechanism, that is the reason we can define a Sensor or Receptors responsible for the ability to smell.